Project Summary/Abstract Elevated platelet counts are detected in about 30% of ovarian cancer patients and associated with a poor prognosis. We found that thrombocytosis in ovarian cancer is not just epiphenomena of an advanced malignancy, but in fact, platelets promote tumor growth. By reducing platelet counts, we reduced the growth of primary tumors in murine models of ovarian cancer. While most of the previous studies focused on the role of platelets in promoting metastasis, we discovered that platelets increase the growth of primary tumors by enhancing proliferation of cancer cells. The basis for the effect of platelets on tumor growth is the interactions between platelets and cancer cells. We found that ovarian cancer cells activate platelets by secreting ADP, and activated platelets secrete Tgf?1 that promotes cancer cell proliferation. Blocking or deficiency of P2Y12 ADP receptors on platelets, blocking or deficiency of Tgf?1 in platelets, or reducing Tgf?1 receptor 1 (Tgf?R1) on cancer cells reduced the pro-growth effects of platelets on ovarian cancer. During studies on ovarian cancer tumors resected from patients and tumor-bearing mice, we observed extravascular platelets inside tumors. We propose that the main effects of platelets on cancer are mediated by extravasated platelets. Migration of platelets outside of blood vessels is not a well-known phenomenon, despite the fact that platelets possess the molecular machinery required for extravasation, and are able to undergo drastic structural changes necessary for extravasation of neutrophils that are professional migratory cells. Our in vivo and in-vitro studies on platelet extravasation into tumors showed that this process is an active process and is reduced by antiplatelet agents (aspirin or ticagrelor). In the first aim of this proposal, we will study the stimuli from tumors that initiate platelet extravasation. We will identify the molecular mechanism of transendothelial migration of platelets, and investigate the facilitatory effects of pericytes on platelets extravasation. After exiting blood vessels, platelets become activated by cancer cells and tumor stroma. In the second aim of this proposal, we will investigate the mechanism of platelet activation in the cancer by dissecting the role of various G-protein-coupled receptors on platelets in tumor growth. We will study the redundancy, synergism, or opposing effects of different platelet G- proteins on platelet-cancer cell interactions. In the third aim, we will investigate the correlation between our findings in the murine models of ovarian cancer and the behavior of ovarian cancer in patients. We have used advanced imaging studies to accurately and objectively quantify platelet density in tumor tissues. We will determine platelet density in tumor specimens resected from 60 patients diagnosed with ovarian cancer in M.D. Anderson Cancer Center (from a pool of 485 patients recruited to the Ovarian Cancer Moon Shots program in our institution). We will correlate platelet density inside tumors with the response rate to surgery, chemotherapy, and antiangiogenic therapy and the survival rates; and determine whether platelet density can be used as a predictive marker for the response rates to therapy and as a prognostic marker for survival and recurrence rates.